1. Field of the Invention
This invention relates generally to apparatus for improving a hearing aid by eliminating oscillations and, more particularly, to apparatus for eliminating such acoustic oscillations by using phase equalization in the signal path.
2. Description of the Background
The typical hearing aid employs a microphone, an amplifier, and a receiver or output transducer located within the ear of the hearing aid wearer. Most modern hearing aids are of the in-the-ear (ITE) type, in which the hearing aid device is located entirely within the wearer's ear. There is another type that is even smaller that is located entirely within the ear canal of the wearer. In all such in-the-ear hearing aids, the audio signals are received near the entrance of the ear canal by the microphone and then amplified and transmitted via a receiver, which performs the electrical-to-acoustical conversion, as sound waves into the ear canal. The amplifier can also be designed to shape the spectral content of the audio signal as required to compensate for the extent of hearing loss of the wearer. In such hearing aids that fit entirely within the ear, it is the practice to provide a vent or passage way through the hearing aid to prevent the wearer from having the feeling of total occlusion that would be brought on by a solid hearing aid filling the ear canal.
A typical hearing aid such as described above is shown in FIG. 1 in which the hearing aid 10 is arranged within the ear canal 12 of the wearer so that audio signals can be received by a microphone 14, amplified in an amplifier 16, and fed to the receiver unit 18. The receiver unit then converts the electrical signals to acoustic signals that impinge on the ear drum 20. The vent is shown at 22, and it is seen that the inner portion of the ear canal 12 is in acoustic communication with the exterior of the hearing aid 10 and, thus, provides a feedback path between the receiver 18 and the microphone 14. In addition, because of the requirement for realistic tolerances in the dimensions of the body of the hearing aid 10, there will be some acoustic leakage between the hearing aid 10 and the ear canal 12, as represented at 24 and 26 in FIG. 1. Thus, an acoustic feedback path exists not only through the vent 22, but, also through the leakage areas 24 and 26 around the hearing aid 10.
Hearing aid research and modern solid-state fabrication techniques have permitted great improvements in the miniaturization of hearing aids, as well as permitting improvement in the overall sophistication of the hearing aid circuitry. In addition, by using such solid-state circuitry, the overall power consumption of a hearing aid has been lessened. Nevertheless, the fundamental problem that severely limits the maximum useable gain that can be provided by the amplifier still remains and that problem is based upon the above-described acoustic feedback. Such acoustic feedback places limits on the maximum usable gain and creates the extremely annoying "howl", which is very irritating to the wearer. Furthermore, the acoustic feedback oscillation alters the overall system response so much that the response at all other frequencies is also significantly degraded.
In analyzing this acoustic oscillation problem, it has been proposed to examine the hearing aid as a control system and FIG. 2 shows a signal flow graph for a typical hearing aid such as shown in FIG. 1. The blocks T.sub.M, T.sub.HA, and T.sub.R shown at 40, 42, 44, respectively, represent the transfer functions for the microphone 14, the amplifier 16, and the receiver 18, respectively. The block T.sub.F shown at 46 represents the transfer function of the acoustic feedback path 22, 24, and 26. Accordingly, in the schematic of FIG. 2 it is understood that the sound input and adder 48 whose output is fed to the input of the microphone transfer function 40, as well as the feedback path transfer function 46 and the output at the receiver transfer function 44, are all actually acoustic paths, whereas electrical signals are represented by the paths between the microphone transfer function 40, the amplifier transfer function 42, and the receiver transfer function 44.
The transfer function of the overall hearing aid including the acoustic feedback as shown in the system of FIG. 2 is given by: ##EQU1##
By defining the open loop transfer function as: EQU T.sub.open =T.sub.M T.sub.HA T.sub.R T.sub.F ( 2)
it is possible to see that when the magnitude of the open loop transfer function is equal to unity and the phase is an integer multiple of 2 .pi., then the system transfer function is undefined and the hearing aid becomes unstable. That is, oscillation will occur.
It has been proposed to reduce the adverse effects of this acoustic feedback by altering either the magnitude or the phase relationships of the feedback-loop of the hearing aid. Phase altering approaches that have been proposed include a frequency shift where the input frequency spectrum of the signal entering the microphone is shifted by a few Hz prior to the amplified signal being fed to the receiver. This approach has been successfully practiced in public address systems for a number of years, however, it has not been successful in hearing aids because of the large percentage variation of the feedback path. On the other hand, the phase information can be altered by providing a time-varying delay in the signal path. This approach can provide a maximum of only 1-2 dB of extra gain and suffers from the further drawback that frequently an audible warbling sound is produced.
In practicing a gain altering technique, the primary purpose is to reduce the gain of the system at the frequency where the oscillations are most likely to occur. Typically, this is accomplished by providing a narrow band notch filter or a comb filter having a number of narrow band notch filters at the frequencies of oscillation. The problem with this approach is that only around 3 to 5 dB of additional usable gain is provided, which is not sufficient for high-gain hearing aids.
Another approach to overcoming this oscillation problem is to provide feedback cancellation in an attempt to cancel the entire effect of the acoustic feedback. Such an approach is represented in FIG. 3, in which an additional feedback path is provided that is intended to be 180.degree. out of phase with the problematic acoustic feedback path. This feedback cancellation is represented in FIG. 3 at block 60 that takes the output of the amplifier block 42 and subtracts it from the output of the microphone block 40 by means of a signal summing block 62. Thus, the intent is to provide a transfer function in block 60 that produces a feedback path equal to, but 180.degree. out of phase with, the acoustic feedback path, as represented by transfer function 46. Although this system does provide some relief from the undesired oscillations other problems are present, such as during normal use the acoustic feedback path changes quite dramatically and if the internal feedback 60 does not adapt to such changes, then the overall hearing aid system is likely to become unstable in any event. This instability is primarily due to the effects of the internal feedback path transfer function 60 itself.
Another problem with the feedback transfer function cancellation system shown in FIG. 3 is because the cancellation is occurring in the complex domain, that is, each transfer function has a real and imaginary part. This means that the precision necessary for the cancellation process for both the real part and imaginary part between T.sub.C and T.sub.M, T.sub.R, T.sub.F must be extremely accurate. Otherwise, a slight disturbances will result in oscillations. Thus, it would appear that this approach requires an adaptive mechanism to identify variations in the feedback transfer function and then to make the necessary changes to the feedback transfer function cancellation element 60. These adaptive algorithms are quite complex and would require a relatively large amount of signal processing power, which makes it impossible to place such a signal processor in an in-the-ear hearing aid.
Upon a slight increase in gain the system becomes quite unstable and peaks appear at the resonant frequencies when the gain is increased only slightly. It is these peaks and instability that are to be eliminated by the present invention.